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Research Aircraft Observations of the Mean and Turbulent Structure of a Low-Level Jet Accompanying a Strong Storm

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  • a Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington
  • | b Northwest Research Associates, Bellevue, Washington
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Abstract

A NOAA P-3 research aircraft collected measurements in a storm off of the U.S. West Coast that featured a low-level jet with wind speeds approaching 50 m s−1. These measurements have been used to document mean and turbulent boundary layer structures in the vicinity of the jet along a pair of transects (∼150 and ∼25 km off the coast). Large magnitudes of turbulent kinetic energy (TKE) and a lack of thermal wind balance were found in the region of the jet, indicating that mixing was an important process. The turbulent fluxes of buoyancy were generally negative, implying that the turbulence near the jet was generated by wind shear. The observed momentum fluxes associated with this wind shear were compared with a TKE-based parameterization for these fluxes. In certain locations, the momentum fluxes were up the vertical wind shear gradient, whereas the parameterization specifies that these fluxes should be down the gradient. Nevertheless, in general the observed and parameterized momentum fluxes agreed favorably, and this agreement was improved with the inclusion of factors related to the magnitude of the TKE and buoyancy effects. The terms in the TKE budget equation were evaluated, revealing substantial variations in shear production and even larger variations in the turbulent transport of TKE. These variations probably are responsible for the highly patchy distribution of TKE in the vicinity of the jet. Because relatively few turbulence measurements have been collected in such strong low-level winds, this case represents a unique opportunity for validating the boundary layer processes simulated by numerical weather prediction models in the storm environment.

Corresponding author address: Nicholas A. Bond, NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115. bond@pmel.noaa.gov

Abstract

A NOAA P-3 research aircraft collected measurements in a storm off of the U.S. West Coast that featured a low-level jet with wind speeds approaching 50 m s−1. These measurements have been used to document mean and turbulent boundary layer structures in the vicinity of the jet along a pair of transects (∼150 and ∼25 km off the coast). Large magnitudes of turbulent kinetic energy (TKE) and a lack of thermal wind balance were found in the region of the jet, indicating that mixing was an important process. The turbulent fluxes of buoyancy were generally negative, implying that the turbulence near the jet was generated by wind shear. The observed momentum fluxes associated with this wind shear were compared with a TKE-based parameterization for these fluxes. In certain locations, the momentum fluxes were up the vertical wind shear gradient, whereas the parameterization specifies that these fluxes should be down the gradient. Nevertheless, in general the observed and parameterized momentum fluxes agreed favorably, and this agreement was improved with the inclusion of factors related to the magnitude of the TKE and buoyancy effects. The terms in the TKE budget equation were evaluated, revealing substantial variations in shear production and even larger variations in the turbulent transport of TKE. These variations probably are responsible for the highly patchy distribution of TKE in the vicinity of the jet. Because relatively few turbulence measurements have been collected in such strong low-level winds, this case represents a unique opportunity for validating the boundary layer processes simulated by numerical weather prediction models in the storm environment.

Corresponding author address: Nicholas A. Bond, NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115. bond@pmel.noaa.gov

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